I. Field of the Invention
The current invention relates to communications. More particularly, the present invention relates to a method and apparatus for calibrating the time delays in Base Stations and Mobile Stations.
II. Description of the Related Art
A modern communications system is required to support a variety of applications. One such communications system is a code division multiple access (CDMA) system that conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wide-Band Spread Spectrum Cellular System,” (commonly referred to as the “IS-95 standard”). In addition, a publication of the Telecommunications Industry Association entitled “The cdma2000 ITU-R RTT Candidate Submission,” which is being developed as TIA/EIA/IS-2000, provides specifications for transmitting data traffic and voice traffic over the forward and reverse links. One method that conforms to this standard for transmitting data traffic in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION,” assigned to the assignee of the present invention and incorporated by reference herein. In accordance with the IS-95 standard, the data traffic or voice data is partitioned into code channel frames that are 20 milliseconds wide with data rates as high as 8×14.4 kbps.
The CDMA system supports voice and data communication between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMobile Station IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention and incorporated herein by reference.
In a CDMA system, communications between users are conducted through one or more Base Stations. In wireless communication systems, forward link refers to the channel through which signals travel from a Base Station to a Mobile Station, and reverse link refers to channel through which signals travel from a Mobile Station to a Base Station. By transmitting data on a reverse link to a Base Station, a first user on one Mobile Station may communicate with a second user on a second Mobile Station. The Base Station receives the data from the first Mobile Station and routes the data to a Base Station serving the second Mobile Station. The first Mobile Station and the second Mobile Station may either be served by a single Base Station or multiple Base Stations, depending upon their location. In any case, the Base Station serving a Mobile Station sends the data to that Mobile Station on the forward link. Instead of communicating with a second user on a Mobile Station, a first Mobile Station may also communicate with a second user on a wireline telephone. The second user is connected to the wireless communication system through a public switched telephone network (PSTN), or a terrestrial Internet through a connection with a serving Base Station.
Up until now, CDMA Base Stations as well as CDMA Mobile Stations have been time calibrated for the sole purpose of providing communication services. With the introduction of position location capabilities, both CDMA Base Stations and Mobile Stations need to be time calibrated for both position location purposes and communications purposes. For communications purposes, accuracy on the order of one CDMA chip (1/1.2288 MHz) is sufficient for proper operation of both the Mobile Stations and the Base Stations. However, for position location purposes, an uncompensated error of one chip corresponds to approximately 300 meters of ranging error.
In communications systems having position location capabilities in which ranging information is derived from both GPS (global positioning system) satellites and Base Stations, timing measurements are made from signals transmitted by both GPS satellites and Base Stations in order to calculate the location of the Mobile Station. GPS signals are broadcast from earth orbiting satellites to earth bound receivers. However, no signals are transmitted from the earth bound receivers back to the GPS satellites. In such systems, GPS receivers calculate their position using pseudo ranges information from satellite-to-earth link only. However, a terrestrial CDMA system is by design a bi-directional communication system with both forward and reverse links. In addition to forward link measurements similar to those used in a GPS system, a measure of Round-Trip-Delay (commonly referred to as “RTD”) is available from a CDMA communication system. RTD is a measurement of the time it takes a CDMA signal to travel from the antenna of a Base Station to a Mobile Station and back to an antenna of the same Base Station. RTD is useful for a positioning system. RTD is calculated in the Base Station for each Mobile Station with which the Base Station is actively communicating.
A CDMA communications system having position location capabilities can be deployed with varying degrees of infra-structure modifications and system performance. One such system would use GPS timing measurements and forward link timing measurements from the terrestrial system, but not RTD. The advantage of this implementation is that Base Station software modifications required to obtain RTD are unnecessary. The disadvantage is a reduction in the availability and accuracy of position location determinations made under adverse GPS and CDMA conditions.
However, regardless of whether the position location capability uses RTD or not, calibration of the Base Station and the Mobile Station is essential to accurate position location determinations.
Therefore, a need currently exists for a method and apparatus for calibrating Base Stations and Mobile Stations in order to allow accurate position location determinations to be made.